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Previous work has shown the complement of leucyl transfer RNA (tRNA(Leu)) isoaccepting species present in soybean cotyledons to change during cotyledon senescence. Of the six tRNA(Leu) species resolved by Freon-column chromatography, the capacity of species 1-4 to accept leucine is decreased between 5 and 21 days of age. Parallel changes in tRNA(Leu) synthetase activity are reported here during the same time period. There is a reduction in the capacity of the synthetase from 21-day-old cotyledons to aminoacylate tRNA(Leu) 1-4, while the capacity to charge tRNA(Leu) 5-6 remains high. When tRNA(Leu) derived from 5-day-old cotyledons is maximally charged with the 21-day synthetase, the subsequent addition of synthetase from 5-day-old cotyledons promotes additional charging, primarily of species 1-4. These results suggest the presence of tRNA(Leu) "subspecies" within the individual peaks resolved on a Freon column, differing in synthetase specificity.
Bick et al. (Fri,) studied this question.